Liquid crystals are used primarily as dielectrics in indicating devices, because the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases) guest/host cells, TN cells having a twisted nematic structure, STN cells ("super-twisted nematic"), SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have good chemical and thermal stability as well as good stability towards electric fields and electromagnetic radiation. Further, the liquid crystal materials should have a low viscosity and in the cells should give short response times, low threshold potentials and a high contrast. Furthermore, at usual operating temperatures of about -30.degree. C. to about +80.degree. C., especially about -20.degree. C. to about +60.degree. C., they should have a suitable mesophase, for example, a nematic or cholesteric mesophase for the aforementioned cells. Further properties, such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible. There has recently been a particular interest in actively addressed liquid crystal indicators, e.g., TFT applications ("thin film transistor") in television sets. The use of cyano compounds having a positive dielectric anisotropy leads, however, in such indicators mainly to an undesired high increase in current.
Since liquid crystals are usually used as mixtures of several components it is important that the components have a good miscibility with one another.